The present invention relates generally to printers, and more particularly to a method and apparatus for detecting and protecting against power short circuits in printers.
An inkjet printer typically prints a plurality of rows of dots during the scanning of a movable print carriage across the print media. The print carriage typically has a plurality of printing devices such as inkjet print cartridges, which include a plurality of printing elements. These printing elements typically include a fluidic structure or chamber and a drop ejection device, typically a piezo-electric actuator or thin film resistor. The printing element typically displaces or boils the ink in the chamber and ejects it through a nozzle onto the print media. These printing elements are displaced relative to each other in the paper axis direction (orthogonal to the scanning axis), allowing the printing of a plurality of rows of dots. After the carriage scans across the media, it is advanced by a number of dot rows corresponding to the number of rows printed by the printhead. Page-wide array printers have fixed printheads and the print media is advanced while the stationary printhead prints.
Modern printers typically include printheads with large arrays of ink ejecting, elements to achieve high throughputs and faster printing speeds. Consequently they require very large power supplies with high current capability to drive each printhead while printing high density images or graphics. For the power supply to maintain a constant voltage while performing high density printing, it must provide high currents. If a short circuit occurs anywhere in the system, the power supply will overdrive the system and damage the drive electronics or print cartridges. Present solutions to this problem include placing short detection devices in the printhead and include placing xe2x80x9ctrip wiresxe2x80x9d in the ink""s path. When a short occurs between the detection and power lines, a register is set that may be read by the printer.
However, short circuits may occur in the printer as well as on the printhead that are not detected by this xe2x80x9ctrip wirexe2x80x9d scheme. Thus, there is a need for a method and system for detecting significant power short circuits that may cause serious damage wherein the detection method and apparatus may be used together with the current xe2x80x9ctrip wirexe2x80x9d schemes to maximize the detection of short circuits and aid in the prevention of damage.
The present invention provides a short circuit detection system, typically for a printer or a printhead, for determining a short circuit therein. The system generally includes a parameter sensing circuit, a parameter usage estimator, and a comparator. The parameter sensing circuit is coupled between a printer supply output and a printhead. The parameter sensing circuit provides a measurement of a selected parameter that may change between the printer power supply and the printhead. The parameter estimator is coupled to receive nozzle firing data and a desired unit parameter, and is used for determining an estimated parameter usage. As used herein, the unit parameter is defined as the unit of the selected parameter per nozzle that is associated with firing a nozzle.
The comparator is coupled to receive the estimated parameter usage and the sensed parameter usage, typically determined between the printer power supply and the printhead. The comparator determines if the sensed parameter usage is greater than the estimated parameter usage, and if so, indicates a short circuit condition.
In one embodiment, the present invention provides a short circuit detection system for determining a short circuit, generally in a printer or printhead. The system may include a voltage or current sensing circuit, a voltage/current usage estimator, and a comparator. The voltage or current sensing circuit is coupled between a printer supply output and a printhead. The voltage or current sensing circuit provides a measurement of the voltage or current that may change between the printer power supply and the printhead. The voltage or current estimator is coupled to receive nozzle firing data and a desired unit voltage or unit current, and is used for determining an estimated voltage or current usage. As used herein, the unit voltage or unit current is defined as the voltage or current per nozzle that is associated with firing a nozzle.
The comparator is coupled to receive sensed voltage or current and the estimated voltage or current. The comparator determines if the sensed voltage or current is greater than the estimated voltage or current, and if the sensed voltage or current is greater than the estimated voltage or current, indicates a short circuit condition. In one implementation, the comparator may send a signal to set an error flag and set a flip-flop or register signal to indicate that a short circuit condition has occurred.
The voltage or current sensing circuit may include a resistor or other sensing device, which is coupled in series between the printer power supply output and the printhead, and which is used for facilitating measurement of a voltage or current across the resistor. The voltage or current sensing circuit also includes an amplifier or buffer that is coupled before and after the resistor. i.e., across the resistor, which is used for determining the voltage or current across the resistor.
In one embodiment, the parameter usage estimator may include a nozzle count logic unit, a unit parameter generator and a parameter multiplier unit. The nozzle count logic unit receives the nozzle firing data and determines how many nozzles will be fired. The unit parameter generator is coupled to receive a desired unit parameter value and to a parameter multiplier unit. The unit parameter generator generates a unit parameter value that is input into the parameter multiplier unit. The parameter multiplier unit is coupled to the nozzle count logic unit and the unit parameter generator, and is used for multiplying the number of nozzles to be fired times the unit parameter value. In addition, the parameter multiplier unit generally adds a predetermined guardband parameter value to provide the estimated parameter usage.
In one embodiment, the printer or printhead short circuit detection system may further include, as a short circuit indicator, a flip-flop, a register, or a flag indicator that is activated by the comparator when if the sensed parameter usage is greater than the estimated parameter usage to indicate that a short circuit has occurred. In addition, where desired, a trip wire system may be coupled to the short circuit indicator to maximize the detection of short circuits.
The present invention includes a method for detecting a short circuit, generally in a printer or printhead. The method includes the steps of: (1) using a sensing circuit for providing a measurement of a selected parameter across the sensing circuit; (2) utilizing a parameter usage estimator for determining an estimated parameter usage; (3) comparing the sensed parameter usage to the estimated parameter usage, and if the sensed parameter usage is greater than the estimated parameter usage, indicating a short circuit condition. In one embodiment, where the sensed parameter usage is greater than the estimated parameter usage, a signal may be sent to set an error flag, or set a flip-flop/register signal to indicate that the short circuit condition has occurred.
In one embodiment, a voltage or current sensing circuit may be implemented as the parameter sensing circuit, and the voltage or current sensing circuit may include a resistor that is coupled in series between a printer power supply output ad a printhead for facilitating measurement of a voltage or current across the resistor and using an amplifier or buffer that is coupled before and after the resistor for determining the voltage or current across the resistor.
Generally, using the parameter usage estimator includes: (1) using a nozzle count logic unit that receives nozzle firing data for determining a number of nozzles to be fired: (2) using a unit parameter generator for generating a unit parameter value; and (3) using a parameter multiplier unit for multiplying the number of nozzles to be fired times the unit parameter value and adding a predetermined guardband parameter value to provide the estimated parameter usage.
The method may further include storing one of: the error flag and information from the flip-flop or register signal to indicate that a short circuit has occurred. In addition, where desired, the step of storing one of the error flag and information from the flip-flop or register signal to indicate that a short circuit has occurred may further include receiving and storing signals from a trip wire system for short circuit detection.
In one embodiment, the present invention includes a method for detecting a short circuit in a printer or printhead. The method includes the steps of: (1) using a voltage or current sensing circuit for providing a measurement of voltage or current VA across a resistor from a printer power supply to the printhead; (2) utilizing a voltage or current usage estimator for determining an estimated voltage or current usage VB; (3) comparing VA and VB to determine if VA greater than VB; and where VA greater than VB, one of: sending a signal to set an error flag and setting a flip-flop or register signal to indicate that a short circuit condition has occurred.
Typically, using the voltage/current sensing circuit includes using the resistor that is coupled in series between the printer power supply output and the printhead for facilitating measurement of a voltage/current across the resistor and using an amplifier/buffer that is coupled before and after the resistor for determining the voltage/current across the resistor.
Generally, using the voltage or current usage estimator includes: (1) using a nozzle count logic unit that receives nozzle firing data for determining a number of nozzles to be fired; (2) using a unit voltage or current generator for generating a unit voltage or current value; and (3) using a voltage or current multiplier unit for multiplying the number of nozzles to be fired times the unit voltage value and adding a predetermined guardband voltage value to provide the estimated voltage/current usage VB.
The method may further include storing one of: the error flag and information from the flip-flop or register signal to indicate that a short circuit has occurred. In addition, where desired, the step of storing one of: the error flag and information from the flip-flop or resistor signal to indicate that a short circuit has occurred may further include receiving and storing signals from a trip wire system for short circuit detection.
In the present invention, the parameter measured may be voltage, current, electric field, power, temperature, or any other selected, desired measurable parameter.
Although the best mode of the short circuit detection system of the present invention is envisioned in the printer carriage, clearly the detection system may be implemented elsewhere in the printing system.